1. Field of the Invention
The invention relates to electric incandescent lamps, and more particularly, to improvements in the fixation of a metal getter element to an incandescent filament.
2. Description of the Prior Art
Despite all precautions taken during the manufacture of incandescent lamps, small amounts of impurities are trapped within the lamp envelope after it is sealed. The impurities may remain on the filament, on the inner wall of the lamp envelope, or in residual gases within the envelope. After sealing, incandescent lamps are typically "flashed" by passing a current through the filament, to crystallize and set the filament. Without appropriate measures, the impurities will react with the hot filament and form vapors which condense on the coolest portions of the lamp envelope and form a black deposit. This deposit is not only cosmetically unsightly, but reduces lamp efficacy. Accordingly, it is common to provide a getter within the lamp envelope to clean-up, or absorb, any impurities which remain after sealing.
There are different types of getters for different types of incandescent lamps. For the typical incandescent lamp which is evacuated, the getter consists of a phosphate material deposited on the filament. When the filament is initially "flashed" the phosphate material deposited thereon is heated and reacts with, or cleans-up, the impurities within the lamp envelope. For evacuated lamps, the amount of impurities is typically small and the phosphate getter is sufficient to adequately clean-up the impurities during flashing, so that no further gettering over lamp life is needed.
Some incandescent lamps are not evacuated but contain a gaseous fill of argon at a pressure of about one atmosphere. With such a fill, the typical phosphate getter used for evacuated lamps is not sufficient to clean up all of the impurities within the lamp. Instead, a getter element consisting of a piece of metal which will clean-up impurities when heated by the filament is secured within the lamp envelope to continuously getter impurities over lamp life. Suitable metals include tantalum, as known from U.S. Pat. No. 3,317,264, and in the case of halogen lamps, copper, as known from U.S. Pat. No. 4,451,760 (Griffin et al).
Some non-evacuated lamps include filament support structure to which the metal getter element can be secured. Other lamps, however, such as radiant heat lamps, have an elongate filament extending between opposing sealed ends of a tubular envelope. In these lamps, there is no frame structure, and the metal getter element must be secured directly to the filament.
U.S. Pat. No. 3,317,264 (Martin et al.) disclose such a lamp in which the getter element is a disc which also supports the filament on the lamp axis. The disc includes a slot that receives the filament and has a thickness selected such that adjacent coil turns of the filament are spread apart and clamp the disc therebetween. Instead of the many discs used in Martin, many commercial lamps employ only one or two of the getter discs and support the filament on the axis with conventional spiral wound wire supports. It has been found, however, that for many combinations of filament pitch and wire diameter, the discs are not adequately secured on the filament, but fall off during the lamp manufacturing process, in shipment, or during use if subject to vibration.
If the getter falls off the filament during or after sealing, the lamp must be scrapped by the manufacturer because the getter will not function unless it is secured to and heated sufficiently by the filament. If the disc falls off after the lamp leaves the factory, the lamp will have a reduced efficiency and a shortened life. It has been found that even if the discs include tooth-like burrs as shown in U.S. Pat. No. 2,980,820 (Brundige et al.), or are manually bent against the coil turns by peening, the scrap rate is still unacceptable. Furthermore, the additional step of peening is labor intensive and stretches the filament coil, forming a cold spot which adversely impacts filament life.
Additionally, the outer edge of the discs are disposed against or very near the inner wall of the lamp envelope. It has been found that reaction of these tantalum getter discs with the lamp impurities upon initial flashing of the filament forms dark rings on the lamp envelope adjacent these edges, which further blacken over lamp life. The dark circles are not cosmetically attractive.
Accordingly, it is the object of the invention to provide an improved metal getter element which reduces lamp cost, through labor savings in installation on the filament and by reducing the scrap rate through better fixation of the getter on the filament, and which reduces unsightly blackening of the lamp envelope.